1. Field of the Invention
The invention relates to a process for manufacturing screens suitable for use in wet screening fibrous paper suspensions. Such screens include at least one screen plate which has a number of sorting apertures.
2. Discussion of Background Information
When processing pulp, screens are used to treat the fibrous suspension in wet screening, e.g., in sorting machines in cellulose and paper manufacturing. Here, in most cases, non-fibrous contaminants are to be retained at a screen because of their size and subsequently eliminated. Here, the fibers can pass through the sorting apertures together with some of the water. Frequently, screens are added to pulpers as well, retaining coarse contaminants and the portion of the paper not yet dissolved. However, there are other applications as well, e.g., in order to fractionate fibrous suspensions according to fiber lengths. In principle, such screens can be flat or curved. They are mostly made of metal, e.g., high-quality steel alloys. They are called screen baskets when they are designed in a rotationally symmetrical way, e.g., cylindrically.
Scrapers passing at close range are generally used to prevent the sorting apertures getting clogged. Their effectiveness can be improved decisively by projecting ridges, because they produce turbulence that prevents solid materials settling.
In many applications, such screens are exposed to considerable wear. This is primarily due to the fact that a certain load of contaminants also arrives in the screen area, e.g., in the case of the above-mentioned use in fibrous paper suspensions. This load of contaminants may comprise metal pieces, broken glass, sand, or stones, for example. Since it is the purpose of the screens to retain this load of contaminants, they cause considerable wear in combination with the passing scraper. Therefore, oblong plating made from wear- resistant materials, e.g., by built-up welding, has been used for quite some time. In other cases, prefabricated wear ridges are mounted onto the screen plate. Therefore, in addition to the above-mentioned increase in turbulence, such ridges or plating can protect the screen from wear. Mounted ridges are known, e.g., from DE 195 06 084 A1. Their production and mounting require higher expenses, particularly because they frequently have wear-resistant, but brittle material.
Even if the screens already serve their purpose, there is always the requirement to reduce the cost of the screens, which must also be considered parts subject to wear, or to improve them. Here, the components in question are actually rather expensive objects that frequently require replacement at certain intervals. Therefore, the manufacturers of such screens are concentrating in particular on producing them as inexpensively as possible.